1. Micelle
A micelle is essentially an aggregate of amphipathic lipid molecules that have both polar or charged groups and non-polar regions dispersed in a liquid. A typical micelle is a sphere in which polar or ionic heads form an outer shell in contact with water, while non-polar tails are sequestered in the interior. Therefore, the core of a micelle resembles an oil droplet.
Amphipathic molecules can form micelles not only in water but also in non-polar organic solvents. The micelle aggregates are named inverse micelles because the situation is inverted with respect to water.
It is important to notice that the length of the non-polar tail, the nature and size of the polar or ionic head, the acidity of the solution, temperature, and the ions in the solution are the most important factors affecting the aggregation. The shape and size of the micelles would change with various situations.
2. Critical Micelle Concentration
The aggregation of micelles only forms when the concentration of the amphipathic lipid reaches a given concentration called the critical micelle concentration (CMC). This condition is monitored by the sudden change in the chemical and physical properties of the solution. In other words, micelles would not form below the CMC.
As shown in the figure, the surface tension would decrease when adding the amphipathic lipid molecules, as the lipids would stay on the surface to reduce the surface free energy. Upon reaching the CMC, additional surfactants would only aggregate to form micelles, and the surface tension would remain relatively constant or change slightly. Therefore, the CMC is the critical point where the surfactants start aggregating to form micelle in solution.
3. Krafft Temperature
Amphipathic lipid molecules would crystallize when the temperature falls below a certain value. The crystal is insoluble, hence the solubility of lipids is decreased. When the temperature is low enough that the maximum solubility of surfactant is lower than the CMC, the whole system would not form micelles in any circumstance. The critical temperature at which the solubility is equal to the CMC is named the Krafft temperature.
References
[1] http://www.whatischemistry.unina.it/en/micella.html
[2] https://en.wikipedia.org/wiki/Micelle
[3] https://en.wikipedia.org/wiki/Critical_micelle_concentration
